In the processing of molten metals, it often is necessary to pump the molten metal from one place to another. When it is desired to remove molten metal from a vessel, a so-called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so-called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so-called gas injection pump is used. In each of these pumps, a rotatable impeller is disposed within the molten metal and, upon rotation of the impeller, the molten metal is pumped as desired. Molten metal pumps of the type referred to are commercially available from Metaullics Systems, 31935 Aurora Road, Solon, Ohio 44139 under the model designation M28-C et al.
In each of the pumps referred to, the impeller is disposed within a cavity formed in a base member. The base member is suspended within the molten metal by means of refractory posts. The impeller is supported for rotation in the base member by means of a rotatable refractory shaft. The base member includes an outlet passageway in fluid communication with the impeller. Upon rotation of the impeller, molten metal is drawn into the impeller, where it then is discharged under pressure through the outlet passageway.
Although the pumps in question operate satisfactorily to pump molten metal from one place to another, certain problems have not been addressed. One of these problems relates to the durability of the drive shaft. Typically the drive shaft is made of a material such as graphite. Graphite is a preferred material for molten metal applications because of its relative inertness to corrosion and also because of its thermal shock resistance. Graphite can be protected from high temperature oxidation and erosion by various sleeves, coatings, and treatments, but it nevertheless deteriorates with time. Another problem with graphite is that it is not very strong, and a graphite drive shaft can be fractured if it is handled roughly or if a large torque load is imposed on the shaft. Desirably, a technique would be found that would increase the longevity of the drive shaft.
Another problem that is not addressed by the pumps in question is that of stirring the molten metal by means of the drive shaft. That is, because the drive shaft rotates in the molten metal, the drive shaft itself stirs the molten metal, causing surface dross formation (metal oxide) which sticks to the shaft and which ultimately can cause imbalance and dynamic failure. Desirably, the molten metal pump would move the molten metal only under the influence of the impeller.
The pumps in question fail to address various other concerns. For example, the pumps are relatively large and heavy, in part because the base member is large, and because the base member must be supported by means of a number of stationary refractory posts. Due to the configuration of the pump, it is difficult or impossible to change the discharge point of the pump relative to the vessel within which the pump is disposed. In the transfer pump embodiment, the outlet portion of the pump sometimes will be broken if the users of the pump do not take proper precautions to avoid undue loading of the outlet. Yet an additional problem relates to difficulties associated in removing the drive shaft and impeller from the pump when replacement of the shaft or the impeller is necessary.